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What is a computer worm?

An essential guide on computer worms and how they actually work

A computer worm is similar to a computer virus, but unlike a virus it does not need to attach itself to an existing program. Instead, it can spread copies of itself from one computer to another without being activated by users.

Designed to spread

Some worms can send themselves to everyone in your e-mail address book - and then to everyone in their e-mail address books, making them capable of spreading to millions of computers within hours. In the most extreme cases, worms have affected computer networks worldwide and drastically slowed down the Internet.

Exploiting caterpillars

Worms typically exploit vulnerabilities in operating systems and spread through e-mail or other file transmission capabilities found on networked computers. They often hide in parts of operating systems that are automatic and usually invisible to you. It is common for worms to be noticed only when their uncontrolled replication consumes considerable system resources, slowing down or halting other tasks.

Worms carrying damage

Many worms have been created only to spread and do not attempt to mess up the computers they pass through. Other worms, however, carry a so-called "payload". A “payload” is a piece of code typically designed to do damage - it might delete files on your PC, encrypt files in an extortion attack, or attach itself to your outgoing e-mail. A very common payload for worms is installing a backdoor which can later be exploited by other malware, allowing the worm author to take control over infected computers.

Notorious worms

Conficker, also known as Downup/Downadup, is a computer worm targeting the Microsoft Windows operating system. It was first detected in November 2008and since then it has spread rapidly into what is now believed to be the largest computer worm infection since the 2003 SQL Slammer, with more than seven million computers infected in over 200 countries.

How to stay safe from computer worms

Worms currently make up almost 8% of all detected malware as shown in reports in early 2011, coming third after the widely spread Trojan infections (almost 70% of all known infections) and traditional viruses (16% of all detected infections), as shown in the following chart.

Because worms spread by exploiting vulnerabilities in operating systems, software vendors supply regular security updates. If you install these updates on your computer, the majority of worms won’t be able to infect it.

Having Anti-spyware, Firewall or Anti Virus software active on your PC will provide protection against all known types of worms, but they must all be kept up-to-date with new virus definitions at least every few days. You also need to be wary of opening unexpected email, running attached files or programs or visiting web sites that are linked to such emails.